Because of their unique catalytic activity and excellent hydrothermal stability, zeolites, aluminosilicate molecular sieves, have been extensively used in petrochemical processing and production of high-value chemicals and biofuels from naturally abundant biomass. Instead of being used as solid Brønsted acid catalysts, molecular sieves containing tetrahedrally coordinated Ti and Sn have been explored as solid Lewis acid catalysts for redox reactions. Ti-containing, high-silica molecular sieves with the zeolite beta topology (Ti-Beta) and MFI topology (TS-1) have been employed for various selective oxidation reactions, such as olefin epoxidation, selective oxidation of alcohols, hydroxylation of phenol and ammoximation of cyclohexanone. Sn-Beta, a tin-containing molecular sieve with the zeolite beta topology, has been used in the Meerwein-Ponndorf-Verley (MPV) reduction of aldehydes and ketones, the Meerwein-Ponndorf-Verley-Oppenauer (MPVO) oxidation of alcohols and the Baeyer-Villiger oxidation reaction. Recently, due to its particular Lewis acidic properties, Sn-Beta has been shown to catalyze the isomerization reactions of triose sugars (dihydroxyacetone and glyceraldehyde), pentose sugars (xylose and xylulose) and hexose sugars (glucose and fructose) with activities that are comparable to biological processes. In particular, it has been revealed that Sn-Beta is a water tolerant Lewis acid catalyst, and can catalyze the isomerization reactions in aqueous phase at low pH, which is most likely due to its hydrophobic nature derived from the high-silica microporous structure. Because of the unique properties, Sn-Beta has also been used for one-pot synthesis of 5-(hydroxymethyl)-furfural (HMF), an important precursor for the production of renewable polymers and biofuels, from glucose by combining with a homogeneous acid catalyst (HCl) in a biphasic system.
Although Sn-Beta has shown promising catalytic properties, its industrial applications and related researches in academia have been hindered by the difficulties in synthesizing this material, particularly the use of hydrofluoric acid and long crystallization time. In general, active Sn-Beta is synthesized using fluoride anion as a mineralizing agent under near-neutral conditions with a crystallization time of around 40 days. The long crystallization time could be due to the relatively low supersaturation degree and limited nucleation caused by fluoride anion and neutral pH used in the synthesis. To reduce the crystallization time, a seeded growth method was applied to the synthesis of Sn-Beta. However, it still requires from 22 days to 30 days to accomplish the synthesis.